1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly, to a direct type backlight unit of an LCD.
2. Description of the Related Art
A cathode ray tube (CRT) is an electrical device for displaying images. CRTs are found in computer monitors, televisions, oscilloscopes, etc. However, CRTs are heavy and bulky and there is a growing demand for slim, light-weight electrical devices.
Other types of display devices have been developed to replace the heavy and bulky CRTs, including LCDs utilizing optical property effects, plasma display panels (PDPs) utilizing plasma discharge effects, and electro luminescence displays (ELDs) utilizing electroluminescence effects.
Recently, thin, light-weight, low power consuming LCDs have been developed. There is a growing demand for LCDs, which are presently developed enough to function as flat display devices in various devices, including laptop computer monitors, desktop computer monitors, and large electronic display boards.
To display an image, LCDs typically transmit light from an external source. Accordingly, most LCDs utilize a backlight unit as an external light source emitting light toward the LCD panel.
Generally, a backlight unit includes one or more tubular light-emitting lamps arranged behind an LCD panel. Depending on the arrangement of the lamps, the backlight units are classified as edge type or direct type units.
In an edge type unit, a lamp unit is installed at a side of a light guide plate. The lamp unit includes a light-emitting lamp, a lamp holder to hold and protect each end of the lamp, and a lamp reflector with one side inserted in a side edge of the light guide plate. The lamp reflector partially encloses the lamp to reflect light from the lamp toward the light guide plate. Edge type backlight units are typically used in smaller LCDs, including laptop computer LCD monitors and desktop computer LCD monitors.
Direct type backlight units have been developed for larger LCDs that are 20-inches or bigger in size. A direct type backlight unit includes a plurality of lamps uniformly arranged below a diffusion plate, emitting light directly to an LCD panel. Because direct type backlight units utilize light more efficiently than edge type backlight units, direct type units are particularly well suited for large sized LCDs requiring more brightness.
However, because direct type backlight units usually operate longer and utilize more lamps than edge type backlight units, direct type backlight units in large LCD monitors and LCD TVs are more prone to fail than edge type backlight units. Moreover, edge type backlight units with lamp units at both sides of a light guide plate are less affected when a lamp unit malfunctions.
Direct type backlight units are more prone to problems because they include a plurality of lamps under a LCD panel. For example, if one of the lamps is not turned on, there is a noticeable difference in brightness on the part of the LCD screen where the malfunctioning lamp is located. Since LCDs with direct type backlight unit require frequent lamp replacements, it is desirable for them to have a structure well-suited for lamp replacements. Regardless, having to replace lamps in either case decreases luminescence efficiency.
Backlight units of the related art will now be described with reference to the accompanying drawings.
FIG. 1 is an exploded perspective view of an edge type backlight unit according to the related art; FIG. 2 is an exploded perspective view of a direct type backlight unit according to the related art.
As stated before, edge type backlight units may be used in desktop or laptop computer monitors. FIG. 1 describes an edge type backlight unit in a desktop monitor. Here the backlight unit includes lamps 10 for a source of light, a light guide plate 11, a diffusion sheet 12, a prism sheet 13, an LCD panel 14 above the prism sheet 13, a fixture (not shown) under the light guide plate 11, and a lower reflection plate 16. The light guide plate 11 guides light from the lamps 10 toward the LCD panel 14, and the diffusion sheet 12 diffuses light from the light guide plate 11 in an upward direction at a predetermined angle. One or more additional diffusion sheets may be included. The prism sheet 13 refracts and condenses the diffused light, sending the light to the LCD panel 14. The lower reflection plate 16 reflects light emitted to the fixture toward the LCD panel 14 to minimize light loss.
An edge type backlight unit further includes lamp reflectors 18 and lamp holders 17. Each of the lamp reflectors 18 encloses a portion of the lamp 10 at a position opposite to the light guide plate 11, such that light emitted in a direction opposite to the light guide plate 11 can be reflected toward the light guide plate 11 to limit loss of light. The lamp holders 17 support the light guide plate 11 and fix their corresponding lamps 10 at a predetermined position.
While the backlight unit for the desktop computer in the present example includes lamps 10 at both sides of the light guide plate 11, backlight units for laptop computers may include a single lamp at one side of the light guide plate.
The direct type backlight unit depicted in FIG. 2 includes a plurality of light-emitting lamps 1, an outer case 3 to fix and support the light-emitting lamps 1, and light scattering units 5a, 5b, and 5c disposed between the light-emitting lambs and an LCD panel (not shown).
The light scattering units 5a, 5b, and 5c prevent contours of the light-emitting lamps 1 from appearing on the LCD panel screen. That is, the light scattering units 5a, 5b, and 5c receive and scatter non-uniform light from the light emitting lamps 1, uniformly projecting the light toward the LCD panel so that the LCD panel can display images with a uniform brightness. To increase the light scattering effect, the light scattering units 5a, 5b, and 5c include a plurality of diffusion sheets and a plurality of diffusion plates.
The direct type backlight unit in FIG. 2 further includes a reflection plate 7 at the inner bottom of an outer case 3 to reflect light from the light-emitting lamps 1 toward the LCD panel and to decrease light loss.
Each of the light-emitting lamps 1 is a cold cathode fluorescent lamp (CCFL). The light-emitting lamp 1 includes a tube and electrodes at both ends. When power is applied to the electrodes, the light-emitting lamp 1 emits light. Each end of the light-emitting lamp 1 is inserted in holes located on opposite sides of the outer case 3.
For each light-emitting lamp 1, lead wires 9 and 9a are connected to electrodes. Each lead wire 9 and 9a is joined to another lead wire by a separate connector (not shown), providing a connection to drive a circuit below the outer case 3.
External electrode fluorescent lamps (EEFL) or hot cathode fluorescent lamps (HCFL) may substitute for the light-emitting CCFL lamps 1 in FIG. 2.
FIGS. 3A to 3C depict representative lamp support structures used in direct type backlight units according to the related art.
In FIG. 3A, a recessed portion 32 is provided to receive an end of a tube type lamp 30. In FIGS. 3B and 3C, the lamp support structures 34 provide an appropriate hole 35 for a particular tube type lamp 30 end.
Because the lamps in FIGS. 3A to 3C have a tubular shape, the radial, light emitting surface of the lamp may be arbitrarily oriented when the lamp is fixed to the lamp support.
This would not be a problem if the light were uniformly emitted from the radial surface of the lamp. However, if light intensity differs depending on the radial direction of the lamp surface, installation of arbitrarily oriented lamps may provide suboptimal performance, including non-uniform brightness and deteriorating image quality. This may be the case when using thin backlight units in which the radial surface of the lamp is treated to produce radially varying light intensities to prevent contours of the lamp from appearing on the LCD panel screen.
In the related art, it is not easy to install a surface treated lamp oriented in a proper direction. To do so, would require a loss in productivity. Moreover, even if the lamp were installed in a proper orientation, the vibration and impact associated with long-term use may alter the orientation.